During a press briefing on the 4th of January, 2022, WHO Incident Manager Dr Abdi Mahamid shared that “the latest studies all pointed to the fact that the Omicron variant affected the upper respiratory system rather than the lungs,” supporting the claim that different variants may have different infection sites. This was warmly received because while variants that infect the lower respiratory tract are associated with lower transmission rates, they also cause “more damage to host tissues.” Conversely, when variants like Omicron infect the upper respiratory tract, inter-host transmission rates tend to be much higher, but it causes less damage to the host. 

Since viruses rely on a host to propagate, harming its host seems like a biological paradox: why would a virus seek to kill its host, if it relies on the host for its own survival? Addressing this question, bacteriologist Theobald Smith (1859-1934) developed the law of declining virulence, proposing that all viruses will eventually adapt to find “equilibrium between themselves and their host.” In other words, viruses will evolve in a way that causes minimal harm to the host, and as a result, allows the virus to thrive in the host for longer. Additionally, it is theoretically beneficial for a virus to cause less damage to its host because it will trigger a weaker immune response than viruses that cause severe damage.

Following the same logic, many postulate that there is an “evolutionary trade-off” between a virus’s transmissibility (how easily a virus can be transmitted from one host to another) and its virulence (the degree of harm a virus inflicts on its host, particularly morbidity and mortality). Therefore, some believe that viruses will either become less transmittable and more virulent, or more transmittable but less virulent—and so far, Omicron suggests that SARS-CoV-2 follows the latter. 

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Indeed, it may appear that the coronavirus has learned to become less deadly in order to survive. But as non-living agents, it is hazardous to assume that viruses, their mutations, and subsequent evolutions result from any survival instincts. While we have seen notable viruses decrease in severity over time, like the H1N1 influenza viruses once responsible for the 1918 Spanish flu and 2009 swine flu pandemics, we have also seen viruses increase in virulence. For example, in 2011, a WHO publication on dengue fever warned that “virulent strains [were] replacing the existing non-virulent strains” of the dengue virus.

With this in mind, many caution against using historical evolutionary trends to anticipate the way coronavirus will evolve. Much remains unknown about the peculiar virus, let alone how it will continue to mutate. There is also a slew of factors that complicate and interfere with Smith’s law of declining virulence, which has been criticised for being naively adaptationist. Rather than argue that natural selection favours viruses that are less virulent overall, a 2020 analysis concluded that “selection likely favours viruses that progress slowly towards disease and whose symptoms remain mild for longer, because such viruses will be transmitted more before people are aware that they are infected.” Thus, factors like the period of possible transmission before the onset of symptoms are also significant considerations.

Ultimately, though the comparably milder symptoms of Omicron may be welcome news, it is by no means an excuse to forego the safety protocols enforced to protect us. If anything, it is a reminder to keep our guard up and stay on our toes.

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Speaking at a COVID-19 media briefing on the 6th of January, WHO director-general Dr Tedros Adhanom Ghebreyesus asserts “while Omicron does appear to be less severe compared to Delta, especially in those vaccinated, it does not mean it should be categorised as mild.” Around the world, economies continue to suffer, frontliners are pushed to their limits, and healthcare systems remain overwhelmed as both Delta and Omicron continue to send patients to the hospital. Moreover, Dr Ghebreyesus stresses the issue of vaccine inequity, which he considers to be one of “the biggest failures” of 2021, describing it as “a killer of people and jobs.” By respecting the protocols put in place, including getting the COVID-19 vaccine and subsequent boosters when able, we can help control the influx of cases and prevent preventable deaths.

“COVID-19 becoming a milder disease is not a decision the virus will make,” proclaims Dr Andrew Pekosz, professor at Johns Hopkins Bloomberg School of Public Health. “It’s a decision that all of us can make if we take advantage of the vaccines that can control spread and reduce the burden of this disease.”

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